Technical Abstract:
Biofortification refers to natural enhancement of the nutritional value of grain or food products. This can be accomplished through traditional breeding and selection for plants that accumulate more nutrients in their edible portions. Since biofortification does not require genetic engineering or synthetic additives, it is acceptable to many consumers. Furthermore, biofortified crops are able to acquire organic certification if grown under organic field conditions. Drs. Shannon Pinson (USDA-ARS Rice Research Unit) and Lee Tarpley (Texas AgriLife Research and Extension Center), both in Beaumont, TX, along with Drs. David Salt and Min Zhang at Purdue University, and Dr. Mary Lou Guerinot at Dartmouth College, have received funding from the National Science Foundation to conduct a research project to identify genes that can be used to enhance the nutritional value of the rice grain. This research includes both increasing the accumulation of nutritional elements such as calcium, zinc, potassium, and iron, and reducing the accumulation of elements that can be detrimental to human and animal health, such as arsenic and cadmium. Because calcium and copper also play a role in protecting plants from pests, diseases, and environmental stresses, genes that increase plant uptake of these elements from soil can also increase the nutritional health of the rice plants, which in turn would increase yield potential and/or stress tolerance. Two rice gene-mapping populations were studied to identify genes affecting the concentrations of 16 elements in brown rice grain. To increase understanding of the grain element genes, one of the populations was grown under two different field conditions — flooded and unflooded. Flooding alters soil chemistry, it converts some elements into forms more available for plant uptake and converts others to less available forms. Flooding also alters the root structure and ability to contact and take up mineral. By including rice grains obtained from plants grown under both flooded and unflooded field conditions, we were able to study how the grain element genes acted the same or differently under two very different environmental conditions. From this study, we identified, altogether, 127 genes associated with the concentrations of individual elements in rice grains. Interestingly, the genes often clustered (mapped to the same chromosomal region) such that there were a total of 40 genomic regions, each associated with up to 11 elements. For example, three elements, cadmium (Cd), magnesium (Mg), and molybdenum (Mo) mapped to the bottom region of chromosome 2, whereas higher on the same chromosome, there is a region associated with 11 different elements (from As (arsenic) to Fe (iron) and S (Sulfur)). The present study does not provide enough detail to determine if these 40 grain-element regions are an actual cluster of multiple genes each affecting the uptake or transport a single element, or if they are instead due to a single gene affecting multiple elements. But it is expected that at least some of these genetic regions associated with multiple elements actually contain a single gene that alters a plant mechanism that affects more than one element. For example, a single gene increasing the number of root hairs, or a gene affecting how roots exude chemicals to alter the soil surrounding them could affect the root uptake of multiple elements. In contrast to the oft-observed gene clustering, one of the copper (Cu) genes we found was not closely linked to genes for other elements. Several genes that specifically affect the uptake and transport of just Cu have been identified in another plant species, Arabidopsis thaliana. It is possible that the Cu-only gene identified here on chromosome 2 is the rice equivalent of one of these Arabidopsis Cu-transporter genes. The present study has identified 40 genomic regions associated with the accumulation of one or more of 16 elements in rice grain, and opens opportunities for a variety of future studies. One planned study will investigate if differences in root architecture are associated with these grain-element genes. Another study will investigate if the Cu-gene identified on rice chromosome 2 is similar to one of the known Arabidopsis Cu-transporter genes. This study represents a major step forward toward the identification of genes that control nutrient availability in rice grain, ultimately enabling the efficient biofortification of rice for multiple mineral nutrients.